1. Field of the Invention
The present invention relates to a rolling mill for metal stocks, and more particularly to a rolling mill which over a wide range can control the plate crown.
2. Description of the Prior Art
In the rolling of a metal stock, such as a steel stock, the work rolls are subjected to a large rolling load which causes a deflection of the rolls, while the portion of the roll surface which is in contact with the rolling stock is forced to be flat. As the rolling thereon proceeds, the edges of the plate being rolled are extended sidewise so that the distribution of thickness of the rolled plate in the direction of the roll axis will be as shown in FIG. 1, leaving the thickest portion in the rolled center of the plate, which gradually becomes thinner toward the plate edges. This is the phenomenon of plate crown usually seen in the conventional rolling mill operation. The amount of the plate crown varies depending on the rolling variables, the thermal expansion of the rolls and the roll wear. It is most desirable that inconsistency in the amount of plate crown in final rolling mill products should be avoided because it will have adverse effects on the product quality and more particularly, the product yield. In FIG. 1 B represents the plate width, tM represents the thickness of the rolled center of the plate, tE represents the thickness of the plate edges, and the plate crown is represented by tM--tE.
In the event that the plate crown as stated above is substantially large and is generated once in the intermediate steps of a rolling operation, trials to correct the crown during the rolling process would be unsuccessful, resulting only in an undesirable wavy shape of the rolled plate, which would cause difficulties in the rolling operation. The resultant plate products are far from satisfaction with respect to the uniform distribution of plate thickness and the quality of plate flatness.
Conventionally, the following measures have been practiced in efforts to reduce or control the plate crown in the mill operation.
(1) A barrel like crown is given beforehand to the roll body.
(2) A concave or convex deflection is given to the upper or lower roll by means of a hydraulic cylinder and the like. This is called a roll bending method.
(3) Combination of the above (1) and (2).
(4) A rolling mill specially designed for controlling the crown is used; for example, in practice a sixhigh mill and a cross mill are used.
However, all of the above measures still have their own defects and disadvantages.
In the case of the measure (1) where the barrel-like crown is given beforehand to the roll body, it is practically impossible to give the roll body a proper crown beforehand which can meet with the rolling variables due to the fact that the width and thickness of the plate vary, the resistance of the plate being rolled to the deformation varies depending on the chemical compositions of the rolling stock and the rolling temperatures, and also the shape of the roll itself changes due to thermal expansion, wear and so on. Moreover, it is extremely difficult to modify and reduce the crown once caused in the plate without problems such as the occurrance of a wavy shape, namely with the assurance of maintaining a satisfactory quality of the plate flatness.
In the case of the measure (2) in which a convex or concave deflection is given to the upper and lower rolls by means of a roll bending device to modify the crown, an excessively large bending force cannot be applied in the conventional plate rolling mill because of the practical limits imposed by the strength of the roll necks, the life of the bearings and so on. Further, as the work rolls and the back-up rolls come into contact with each other along almost the entire length of the rolls, and the work rolls are subjected to an excessive moment of bending by the back-up rolls on the marginal portions outside the edges of the plate being rolled, the bending effect on the work rolls are off-set so much. For these reasons, the bending effect on the work rolls is prevalent only along and in the vicinity of the plate edge portions, and thus no substantial effect can be obtained upto the rolled center of the plate being rolled.
Also in the case of the measure (3) which combines the measures (1) and (2), no satisfactory controlling effect on the plate crown can be obtained, thus failing to withstand changes of the plate crown due to a variation in the resistance to deformation of the rolling stock, variation in the plate width, thermal expansion of the rolls, and roll wear as explained in connection with the measure (1).
In the case of the measure (4), a marked controlling effect can be obtained, but the problem in this case is that the capital cost is enormously high and it would be quite difficult to modify the existing mills so as to meet this purpose.
In order to reduce or control the plate crown according to the measure (1), as disclosed in Japanese Patent Publication No. Sho 59-35283, a large convex crown is given to the back-up rolls 2, 2' in contact with the work rolls 1, 1' as shown in the accompanying FIG. 2, whereby the work rolls are deformed by the bending moment as caused by the contact load imposed by the back-up rolls so that the rolling stock is prevented from being substantially reduced at the edge portions.
Also the roll bending effect by the roll bending device 4, 4' is remarkable because the edges of the work rolls are not restrained as clearly disclosed, for example, in Japanese Patent Application Nos. Sho 49-84209 and 50-18864. However, in actual rolling mill practices, various widths of plates are rolled, so it is very important to guarantee the desired bending effect as mentioned above for any width of plate. FIG. 3 shows the controllability range of plate crown defined by roll deformation under uniform force (herein called the controllability range of plate crown under uniform force) obtainable by various crown values given to the back-up rolls.
The term "the plate crown under uniform force" herein used means a calculated plate crown from the plate thickness distribution defined by the shape of work rolls determined by the deformation of the rolls caused by the load acting on the rolls imposed by the mill stand, which load is assumed to be distributed uniformly across the width of the plate being rolled. This calculated plate crown is not identical to the actual plate crown, but is very useful for relative comparison of the plate crown controllability of rolling mills.
While a wider controllability range of plate crown under uniform force can be obtained by a larger convex crown given to the back-up rolls, the ordinary crown control cannot be achieved if the plate width is too large, because the plate crown controllability range shifts to the minus side (concave crown), as shown in FIG. 3.
Due to the above problem, a large convex crown could not conventionally be given to the back-up rolls. According to the results of one trial in giving a small convex crown of about 1.0 mm to the back-up rolls, the controllability range of plate crown under uniform force could not cover all of the variable widths of plates, thus substantially limiting the practical plate crown control.
Although it is also known to give a convex or concave crown shape of about 0.1 mm in diameter to the work rolls, it has never hitherto been known to correlate the crown amount given to the work rolls with the crown amount given to the back-up rolls.
The problems of the prior art of giving the crown only to the work rolls have already been discussed.